Pressure Sensor Device and Method for Forming a Pressure Sensor Device

ABSTRACT

In an embodiment a method for forming a pressure sensor device includes providing a pressure sensor on a substrate body, the pressure sensor comprising a membrane, depositing a top layer on top of the substrate body and the pressure sensor, connecting a cap body with the top layer, a mass of the cap body being approximately equal to a mass of the substrate body and introducing at least one opening in the cap body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.16/333,671, entitled “Pressure Sensor Device and Method for Forming aPressure Sensor Device”, which was filed on Mar. 15, 2019, which is anational phase fling under section 371 of PCT/EP2017/074953, filed Oct.2, 2017, which claims the benefit of European patent application16191894.1, filed on Sep. 30, 2016, all of which are incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The present application relates to a pressure sensor device and a methodfor forming a pressure sensor device.

BACKGROUND

Pressure sensors are sensitive to stress and therefore need to beprotected from undesired stress which can be caused, for example, bymechanical deformation. If a pressure sensor is arranged on a chipstress can be induced due to different coefficients of thermal expansionof different parts of the chip. Also, if the processing of the pressuresensor involves heating and solidification steps, additional stress canbe exerted on the pressure sensor. As another example, if a pressuresensor is arranged within a device, mechanical forces of the device canoccur and induce stress on the pressure sensor. It is desirable tomaintain the conditions of the calibration of the pressure sensor, thismeans additional stress should be avoided in order to guarantee acorrect pressure reading of the pressure sensor.

Additional stress induced on a pressure sensor can be avoided, forexample, by the use of compliant layers which can be silicone-based.Such layers can be employed in land grid array packages. However, theuse of compliant layers increases the total thickness of the device andthe packages of the device are usually larger than the footprint of thepressure sensor.

SUMMARY

Embodiments provide a pressure sensor device with an increased accuracyin pressure sensing. Further embodiments provide a method for formingsuch a pressure sensor device with an increased accuracy.

In one embodiment of the pressure sensor device, the pressure sensordevice comprises a substrate body. The substrate body can be a wafer, asubstrate or bulk material and it can comprise silicon or glass. Thesubstrate body can comprise a complementary metal oxide semiconductordevice and a substrate. The pressure sensor device further comprises apressure sensor comprising a membrane. The pressure sensor is arrangedon top of the substrate body, for example on top of the complementarymetal oxide semiconductor device. The pressure sensor can also comprisea cavity below the membrane.

In one embodiment, the pressure sensor device comprises a cap bodycomprising at least one opening. The cap body can comprise silicon orglass and can be of the same material as the substrate body. It is alsopossible that the cap body and the substrate body comprise differentmaterials. The cap body and the substrate body can comprise materialswith similar coefficients of thermal expansion. The cap body can be awafer, a substrate or a bulk material which is arranged on top of thepressure sensor. This means the cap body is arranged on the side of thepressure sensor which faces away from the substrate body. The opening inthe cap body can be formed by deep reactive ion etching in combinationwith grinding. The diameter or a lateral extension of the opening can,for example, amount to approximately 800 μm. Advantageously, thediameter or the lateral extension of the opening is small in comparisonto the lateral extension of the cap body.

In one embodiment of the pressure sensor device, the pressure sensor isarranged between the substrate body and the cap body in a verticaldirection which is perpendicular to the main plane of extension of thesubstrate body and the mass of the substrate body equals approximatelythe mass of the cap body. The main plane of extension of the substratebody extends in a lateral direction of the substrate body. The thicknessof the substrate body is given in the vertical direction and thethickness of the substrate body is small in comparison to the lateralextension of the substrate body. This means, the vertical directioncorresponds to a stacking direction of the pressure sensor device. Thismeans, the pressure sensor is arranged on top of the substrate body instacking direction and the cap body is arranged on top of the pressuresensor in stacking direction. The mass of the substrate body can, forexample, amount to at least 80% of the mass of the cap body and at most120% of the mass of the cap body. Optionally, the mass of the substratebody can amount to at least 90% of the mass of the cap body and at mostno % of the mass of the cap body. Optionally, the mass of the substratebody can amount to at least 95% of the mass of the cap body and at most105% of the mass of the cap body. This means, the masses of thesubstrate body and the cap body are approximately equal within atolerance of manufacturing.

If the masses of the substrate body and the cap body are approximatelyequal, the pressure sensor or the membrane of the pressure sensor arelocated in a plane of minimal mechanical stress change when subjected toexternally applied, mechanical or thermal induced deformations. Thismeans, there is a plane of minimal stress within the device.Advantageously, the membrane of the pressure sensor is located in theplane of minimal stress within the pressure sensor device.

The cap body can be patterned before it is attached on the pressuresensor and the substrate body. For example, a recess can be formed inthe cap body so that the cap body is not in direct contact with themembrane of the pressure sensor. In order to keep the masses of thesubstrate body and the cap body approximately equal, the cap body needsto be thicker than the substrate body because of the opening in the capbody. The opening of the cap body can have a lateral extension ofapproximately one third of the lateral extension of the cap body. Inthis case, the cap body needs to be thicker than the substrate body inorder to position the pressure sensor in the plane of minimal stress ofthe pressure sensor device. It is also possible that the lateralextension of the opening of the cap body is larger than the lateralextension of the membrane of the pressure sensor. Therefore, thethickness of the cap body depends on the lateral extension of theopening in the cap body.

If the substrate body is transparent for light or electromagneticradiation, an optical sensor can advantageously be integrated in thedevice next to the pressure sensor.

By positioning the pressure sensor in the plane of minimal stress, it isprotected against strain which can be caused, for example, by differentcoefficients of thermal expansion of the pressure sensor device and anunderlying printed circuit board on which the pressure sensor ismounted. The pressure sensor can also be protected against additionalstress induced from other parts of the pressure sensor device or thesurroundings of the pressure sensor device. If the additional stress onthe pressure sensor is minimized by placing the pressure sensor in theplane of minimal stress, the pressure sensor can operate under the sameconditions as during the calibration of the pressure sensor, thusguaranteeing a correct and more accurate pressure reading.

Furthermore, by positioning the cap body on top of the pressure sensor,the thickness of the pressure sensor device is increased such that thesubstrate body can be thin enough to allow a processing from a bottomside of the substrate body, where the bottom side of the substrate bodyfaces away from the pressure sensor. This means that through siliconvias can be etched in the substrate body to electrically contact thepressure sensor from the backside of the pressure sensor facing awayfrom the cap body. Moreover, it is possible to release the membrane ofthe pressure sensor after the processing steps which can comprise theetching of the through silicon vias which avoids a potential membranefracture during the processing steps. Additionally, the employment of acap body and a substrate body allows an efficient packaging of thepressure sensor since no compliant or glue layers are required for thedecoupling of the membrane. Therefore, the thickness of the packagingcan be reduced and also the footprint of the packaging. The thickness ofthe packaging can for example amount to 300 to 600 μm and the footprintof the packaging can amount to 1 to 2 mm². Furthermore, the assemblycosts of the packaging can be reduced.

In one embodiment the pressure sensor device comprises a substrate body,a pressure sensor comprising a membrane, and a cap body comprising atleast one opening. The pressure sensor is arranged between the substratebody and the cap body in a vertical direction which is perpendicular tothe main plane of extension of the substrate body, and the mass of thesubstrate body equals approximately the mass of the cap body.

In one embodiment of the pressure sensor device, the mass of thesubstrate body amounts to at least 95% of the mass of the cap body andat most 105% of the mass of the cap body. This means the masses of thecap body and the substrate body are approximately equal so that thepressure sensor is located in the plane of minimal stress of thepressure sensor device. It is also possible that the mass of thesubstrate body amounts to at least 80% of the mass of the cap body andat most 120% of the mass of the cap body. Optionally, the mass of thesubstrate body amounts to at least 90% of the mass of the cap body andat most no % of the mass of the cap body.

In one embodiment of the pressure sensor device, the pressure sensorcomprises a capacitive pressure sensor comprising a cavity below themembrane. The pressure sensor can be formed, for example, by providing asacrificial layer above a bottom electrode and electrically conductivevias which are arranged around the area which is supposed to be thecavity of the pressure sensor. A top electrode can be deposited on topof the sacrificial layer and the electrically conductive vias, and thetop electrode can be patterned to form the membrane. The sacrificiallayer is etched away through etch holes in the membrane such that thecavity is formed below the membrane. The membrane can be sealed byplasma-enhanced chemical vapor deposition of silicon nitride, which canbe silicon-rich, or silicon nitride and silicon oxide such that acompressive film is formed on top of the membrane. Advantageously, acompressive film is less permeable for gases from the surroundings intothe cavity.

In one embodiment of the pressure sensor device, the substrate bodycomprises at least one vertical electrically conductive via and/or thepressure sensor device is surface mountable. Vertical in this case meansalong the vertical direction which means that the electricallyconductive via extends from the bottom side of the substrate body whichfaces away from the pressure sensor in the direction of the pressuresensor. The electrically conductive via electrically contacts thepressure sensor or another electrical contact, for example a contact ofa complementary metal oxide semiconductor device. The electricallyconductive via can be a through silicon via which is formed bypatterning of the substrate body, for example by etching. A trenchpatterned in the substrate body can be coated with an isolation materialand with an electrically conductive material to contact the pressuresensor.

The electrically conductive material of the electrically conductive viacan comprise titanium and/or titanium nitride and tungsten or tantalumand/or tantalum nitride and copper. In order to form the electricallyconductive via in the substrate body, the substrate body can be thinnedto, for example, 100 to 200 μm. Since the cap body is arranged on top ofthe pressure sensor, it is possible to thin the substrate body so thatthe electrically conductive via can be formed. With this, the pressuresensor device can be surface mountable. Surface mountable means that thepressure sensor device can be electrically contacted from the bottomside of the substrate body. The electrically conductive via can beelectrically contacted on the bottom side of the substrate body by asolder ball.

In one embodiment of the pressure sensor device, the pressure sensor ispositioned on top of an integrated circuit. The integrated circuit canbe a complementary metal oxide semiconductor device. With this, thetotal size of the device can be reduced since the pressure sensor isarranged on top of the integrated circuit.

In one embodiment of the pressure sensor device, a top layer covers thepressure sensor on the side of the pressure sensor facing the cap body,and the top layer and the cap body are connected via direct bonding. Thetop layer can be deposited onto the pressure sensor before the membraneis released. The top layer can, for example, comprise a bond oxide andit can be, for example, 2 to 3 μm thick. The top layer can comprisesilicon dioxide and/or silicon nitride. The advantages of direct bondingare that a very stiff connection can be formed between the cap body andthe top layer and that the bond is compatible with high temperatureswhich can employed during the processing of the pressure sensor device.

For a stiff connection, it is important that the surface of the toplayer is topographically flat. Therefore, the surface of the top layercan be flattened by chemical mechanical polishing before connecting withthe cap body. A good mechanical coupling and stiffness of the connectionbetween the top layer and the cap body is important for the stresscompensation in the plane of the membrane of the pressure sensor. If theconnection between the top layer and the cap body is not stiff enough,strain cannot be transferred to the cap body and the plane of minimalstress is less well defined. Therefore, the stiffness of the connectioncan also influence the required thicknesses of the cap body and thesubstrate body. It is also possible that the top layer and the cap bodyare connected via a glue.

If the top layer comprises a metal it can be connected with the cap bodyby eutectic bonding. A eutectic bond is a very stable bond and it isalso stable at high temperatures. Therefore, in the case of eutecticbonding other processing steps, such as for example the formation of anelectrically conductive via with tungsten in the substrate body, arepossible.

In one embodiment of the pressure sensor device, the top layer comprisesat least one electrically conductive wall which is arranged on top ofthe pressure sensor surrounding the opening and which is in directcontact with the pressure sensor and the cap body. The top layer cancomprise one or more electrically conductive walls. The electricallyconductive wall is arranged to protect the top layer against the etchingof the cavity such that the top layer is not etched away where it isarranged between the cap body and the substrate body. This means theelectrically conductive wall is arranged around the membrane of thepressure sensor and around the area where the cavity is etched. Withthis, it is avoided that the top layer is etched away below the capbody. Therefore, the electrically conductive wall can be formed beforethe membrane is released. The electrically conductive wall can alsoserve to stabilize the pressure sensor device.

In one embodiment of the pressure sensor device, the opening in the capbody is positioned above the pressure sensor in vertical direction andextends over the total lateral extension of the pressure sensor. Theopening is introduced into the cap body so that the pressure sensor canmeasure the pressure of the surroundings of the pressure sensor device.Therefore, the membrane with a sealing needs to be in direct contactwith the air or the gas from the surroundings of the pressure sensordevice.

The lateral extension of the opening can be the same as the lateralextension of the pressure sensor or the lateral extension of the openingcan be larger than the lateral extension of the pressure sensor. Thelateral extension refers to an extension in two dimensions which aregiven by two directions which are perpendicular to the verticaldirection. The opening can be shaped circular. It is also possible tointroduce more than one opening in the cap body.

The cap body can be thinned before the opening is introduced. Thethickness of the cap body depends on the size of the opening. This meansif the lateral extension of the opening is small, the cap body can bethinner in order to keep the plane of minimal stress in the plane of themembrane.

In one embodiment of the pressure sensor device, the opening in the capbody is positioned above the pressure sensor in vertical direction andthe lateral extension of the opening is smaller than the lateralextension of the pressure sensor. It is advantageous to keep the lateralextension of the opening small and, therefore, also the thickness of thecap body in order to reduce the total height of the pressure sensordevice. It is also possible that the opening is not positioned above themembrane in vertical direction but besides the membrane in verticaldirection.

Furthermore, a method for forming a pressure sensor device is provided.The pressure sensor device may be produced by means of one of themethods described here. This means all features disclosed for thepressure sensor device are also disclosed for the method for forming apressure sensor device and vice-versa.

According to at least one embodiment of the method for forming apressure sensor device, the method comprises providing a pressure sensoron a substrate body, the pressure sensor comprising a membrane, anddepositing a top layer on top of the substrate body and the pressuresensor. The method further comprises connecting a cap body with the toplayer, the mass of the cap body being approximately equal to the mass ofthe substrate body, and introducing at least one opening in the capbody.

The mass of the substrate body can, for example, amount to at least 95%of the mass of the cap body and at most 105% of the mass of the capbody. It is also possible that the mass of the substrate body amounts toat least 80% of the mass of the cap body and at most 120% of the mass ofthe cap body. Optionally, the mass of the substrate body amounts to atleast 90% of the mass of the cap body and at most no % of the mass ofthe cap body. This means, the masses of the substrate body and the capbody are approximately equal within a tolerance of manufacturing, suchthat the pressure sensor is positioned in the plane of minimal stress.

The pressure sensor can be a capacitive pressure sensor with a membraneand a cavity. The substrate body can comprise silicon or glass, and itcan also comprise a complementary metal oxide semiconductor device. Thetop layer can be a bond oxide, for example silicon dioxide or siliconnitride, which needs to be topographically flat. The cap body and thetop layer can be connected by direct bonding, via gluing or via eutecticbonding. The opening can be introduced in the cap body by deep reactiveion etching or by grinding. It is also possible that the cap wafer ispatterned before connecting with the top layer, for example with arecess for the membrane so that the cap body is not in direct contactwith the membrane.

By positioning the pressure sensor in the plane of minimal stress, it isprotected against strain which can be caused, for example, by differentcoefficients of thermal expansion of the cap body and the substratebody. The pressure sensor can also be protected against additionalstress induced from other parts of the pressure sensor device or thesurroundings of the pressure sensor device. If the additional stress onthe pressure sensor is minimized by placing the pressure sensor in theplane of minimal stress, the pressure sensor can operate under the sameconditions as during the calibration of the pressure sensor, thusguaranteeing a correct and more accurate pressure reading.

According to at least one embodiment of the method for forming apressure sensor device, the mass of the substrate body amounts to atleast 95% of the mass of the cap body and at most 105% of the mass ofthe cap body. It is also possible that the mass of the substrate bodyamounts to at least 80% of the mass of the cap body and at most 120% ofthe mass of the cap body. Optionally, the mass of the substrate bodyamounts to at least 90% of the mass of the cap body and at most no % ofthe mass of the cap body. This means the masses of the cap body and thesubstrate body are approximately equal so that the pressure sensor islocated in the plane of minimal stress of the pressure sensor device.

According to at least one embodiment of the method for forming apressure sensor device, a handling wafer is connected to the substratebody at the bottom side of the substrate body by an adhesive materialwhich can be removed. Furthermore, the handling wafer and the adhesivematerial are removed. The handling wafer can be connected to thesubstrate body in order to protect electrically conductive vias in thesubstrate body from mechanical damage or damage from an etching step.For example, during the thinning of the cap body or the etching of theopening, electrically conductive vias in the substrate body can beprotected by the handling wafer. The adhesive material can behigh-temperature compatible and it can be removed. These features areadvantageous for the processing of the pressure sensor device since insome processing steps, high temperatures can be required. If only lowertemperatures are required during processing, it is also possible toemploy a temporary bonding or tape as the adhesive which is compatibleonly to low temperatures and which can be removed.

According to at least one embodiment of the method for forming apressure sensor device, a handling wafer is connected to the substratebody at the bottom side of the substrate body by an adhesive materialwhich can be patterned. Furthermore, the handling wafer is removed andthe adhesive material is not removed. Advantageously, the adhesivematerial is compatible with high temperatures. If the adhesive materialcannot be removed, it is required that it can be patterned such that thepressure sensor and also, for example, a complementary metal oxidesemiconductor device can be electrically contacted from the bottom sideof the substrate body.

According to at least one embodiment of the method for forming apressure sensor device, a vertical electrically conductive via is etchedin the substrate body before the membrane is released. With the verticalelectrically conductive via, the pressure sensor can be electricallycontacted and also, for example, other devices in the pressure sensordevice. If the vertical electrically conductive via is etched in thesubstrate body before the membrane is released, the membrane isprotected by the sacrificial layer during the etching of theelectrically conductive via and other processing steps. Therefore,potential membrane fracture during processing is avoided.

According to at least one embodiment of the method for forming apressure sensor device, a vertical electrically conductive via is etchedin the substrate body after the membrane is released. This means themembrane is released before further processing which can be the bondingof the cap wafer to the top layer. With this, it is possible to keep theopening in the cap body small since it is not required to etch thecavity through the opening of the cap body.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of figures may further illustrate and explainexemplary embodiments. Components that are functionally identical orhave an identical effect are denoted by identical references. Identicalor effectively identical components might be described only with respectto the figures where they occur first. Their description is notnecessarily repeated in successive figures.

FIG. 1 shows a cutaway view of an exemplary embodiment of the pressuresensor device;

FIG. 2 shows a schematic cutaway view of a pressure sensor device on aprinted circuit board;

FIG. 3 shows the stress level as a function of cap body thickness;

With FIGS. 4A to 4O an exemplary embodiment of the method for forming apressure sensor device is described;

With FIGS. 5A to 5L another exemplary embodiment of the method forforming a pressure sensor device is described;

FIG. 6 shows an exemplary embodiment of the pressure sensor device withelectrically conductive walls;

FIG. 7 shows an exemplary embodiment of the pressure sensor device witha reduced opening in the cap body;

FIGS. 8 and 9 show exemplary embodiments of the pressure sensor devicewith a reduced opening in the cap body and a sealing on the membrane;and

With FIGS. 10A to 10K another exemplary embodiment of a method forforming a pressure sensor device is described.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a schematic cutaway view of an exemplary embodiment of thepressure sensor device 10 where the plane of minimal stress is indicatedby the dashed line. On top of a substrate body 11 a pressure sensor 12with a cavity 16 and a membrane 13 is arranged. The pressure sensor 12is electrically contacted by two electrically conductive vias 17. Theelectrically conductive vias 17 extend through the substrate body 11from a bottom side 18 of the substrate body 11 towards the pressuresensor 12. An isolation material 19 electrically isolates theelectrically conductive vias 17 from each other and from the substratebody 11. On top of the pressure sensor 12 a top layer 20 is positionedwhich connects the pressure sensor 12 with a cap body 14. Thus, thepressure sensor 12 is arranged between the substrate body 11 and the capbody 14 in a vertical direction z. The cap body 14 and the top layer 20can be connected via direct bonding. The cap body 14 comprises anopening 15 which is arranged above the pressure sensor 12. The plane ofminimal stress is approximately located in the plane of the pressuresensor 12 and advantageously in the plane of the membrane 13 of thepressure sensor 12. In order to place the plane of minimal stress in theplane of the membrane 13 the mass of the substrate body 11 equalsapproximately the mass of the cap body 14. In other embodiments, thismeans for example that the mass of the substrate body 11 amounts to atleast 95% of the mass of the cap body 14 and to at most 105% of the massof the cap body 14.

FIG. 2 shows a schematic cutaway view of a pressure sensor device 10mounted onto a printed circuit board. The pressure sensor device 10 withthe printed circuit board is schematically depicted as one device body44. Due to for example different coefficients of thermal expansion ofthe pressure sensor device and the printed circuit board, the devicebody 44 is bent in such a way that the top part is compressed and thebottom part experiences a tensile stress. The tensile and thecompressive stresses are indicated by arrows. In one plane through thedevice body 44 the stress is minimal which is indicated by the dashedline.

FIG. 3 shows the stress level y as a function of cap body thickness xfor a constant substrate body thickness. The additional stress isintroduced on the pressure sensor device 10 by soldering the pressuresensor device 10 onto a printed circuit board. The optimal thickness xof the cap body 14 is at zero stress. The stress change is plotted fortwo different diameters of the opening 15 in the cap body 14. In thecase of the straight line the diameter of the opening 15 amounts to 500μm and in the case of the dashed line the diameter of the opening 15amounts to 250 μm. The inset in FIG. 3 shows a schematic of a pressuresensor device 10 with the opening 15 in the cap body 14.

With FIGS. 4A to 4O an exemplary embodiment of the method for forming apressure sensor device 10 is described. In the process flow describedwith FIGS. 4A to 4O the membrane 13 of the pressure sensor 12 isreleased before the electrically conductive via 17 is formed in thesubstrate body 11.

FIG. 4A shows the substrate body 11 which comprises an integratedcircuit 21 which can be for example a complementary metal oxidesemiconductor device. The integrated circuit 21 is arranged on top of asubstrate 43 which is comprised by the substrate body 11 and which cancomprise silicon. On top of the integrated circuit 21 the pressuresensor 12 is arranged. By arranging the pressure sensor 12 on top of theintegrated circuit 21 instead of next to it, the footprint of the deviceis reduced. In this exemplary embodiment the pressure sensor 12comprises a bottom electrode 22 on which a dielectric etch stop layer 23is arranged. A sacrificial layer 24 is deposited on top of thedielectric etch stop layer 23. In order to form the cavity 16 of thepressure sensor 12, the sacrificial layer 24 is patterned in such a waythat trenches are formed in the sacrificial layer 24. The trenches arefilled with an electrically conductive material 34 as for exampletungsten. In order to form the membrane 13, an electrically conductivelayer 25 is deposited on top of the sacrificial layer 24. Below theelectrically conductive layer 25 there is an adhesion layer 26 and thereis another adhesion layer 27 on top of the electrically conductive layer25. The electrically conductive layer 25 and the adhesion layers 26 and27 comprise several etch openings 28. The bottom electrode 22 of thepressure sensor 12 is electrically connected with the integrated circuit21 by electrically conductive walls 29. The integrated circuit 21comprises a back contact 30.

As shown in FIG. 4B, in the next step of the method for forming apressure sensor device 10, a top layer 20 is deposited on top of thepressure sensor 12. The top layer 20 can comprise a bond oxide and canbe for example 2 to 3 μm thick. Furthermore, additional electricallyconductive walls 29 are formed around the membrane 13. The furtherelectrically conductive walls 29 are also connected by an electricallyconductive layer 25 around which adhesion layers 26 and 27 are arranged.A mask 31 is positioned on top of the top layer 20 in order to patternthe top layer 20 above the additional electrically conductive walls 29.

In the next step of the method, as shown in FIG. 4C, the top layer 20 ispatterned in such a way that electrically conductive walls 29 are formedaround the membrane 13 in the top layer 20. The electrically conductivewalls 29 can be filled with tungsten. After the deposition of thematerial of the electrically conductive walls 29 the surface of the toplayer 20 is planarized for example by chemical mechanical polishing. Inorder to achieve a stiff connection between the top layer 20 and the capbody 14 it is necessary that the surface of the top layer 20 istopographically flat.

FIG. 4D shows that a mask 31 is positioned on the top layer 20. The mask31 should be able to withstand an etching step with for example hydrogenfluoride vapor. In the next step, the mask 31 is patterned in such a waythat the area above the membrane 13 on the pressure sensor 12 is free ofthe mask 31.

FIG. 4E shows that in the next step of the method the sacrificial layer24 below and around the membrane 13 is etched away. With this, themembrane 13 of the pressure sensor 12 is released. Also, parts of thetop layer 20 around the membrane 13 are etched away. The electricallyconductive walls 29 and the additional electrically conductive walls 29act as an etch stop so that not the whole top layer 20 is etched away.

As shown in FIG. 4F in the next step a sealing layer 32 is deposited ontop of the membrane 13 and the mask 31. The sealing layer 32 cancomprise silicon nitride. The sealing layer 32 is deposited at elevatedtemperatures as for example temperatures above 400° C. by plasmaenhanced chemical vapor deposition, and therefore after cooling down thesealing layer 32 is compressive. With this, the sealing layer 32 is lesspermeable for gases from the surroundings into the cavity 16.

In FIG. 4G it is shown that another top layer 20 is deposited on top ofthe sealing layer 32. The other top layer 20 can also comprise a bondoxide. The other top layer 20 is deposited in order to improve thebonding strength between the top layer 20 and the cap body 14. Thesurface of the top layer 20 needs to be topographically flat in order toachieve a stiff connection to the cap body 14.

As shown in FIG. 4H in the next step of the method the cap body 14 isconnected with the sealing layer 32. It is also possible that anothertop layer 20 is arranged between the sealing layer 32 and the cap body14 in order to achieve a stiff connection. The cap body 14 are connectedwith the sealing layer 32 or the top layer 20 at elevated temperatureswhich could be for example around 450° C. As shown in FIG. 4H thesealing layer 32 on top of the membrane 13 is not in direct contact withthe cap body 14. Therefore, the membrane 13 is still released and noadditional stress is imposed on the membrane 13 from the cap body 14.With the cap body 14 being connected with the pressure sensor 12 and thesubstrate body 11, the pressure sensor device 10 is turned upside downand the substrate 43 is thinned to a required thickness by for examplegrinding. A required thickness of the substrate 43 can be 100 to 200 μm.

FIG. 4I shows that the substrate body 11 is patterned. This means atrench is formed in the substrate 43, for example by deep reactive ionetching. The trench extends through the whole substrate 43 in verticaldirection z from the bottom side 18 to the integrated circuit 21. In anext step the oxide on the bottom of the integrated circuit 21 is alsoetched away such that the back contact 30 of the integrated circuit 21can be electrically contacted. In the next step an isolation layer 33 isdeposited on the bottom side 18 of the substrate body 11 and within thetrench in the substrate 43.

As shown in FIG. 4J, in the next step of the method the isolation layer33 is etched within the trench such that the back contact 30 of theintegrated circuit 21 is free of the isolation layer 33. For etching theisolation layer 33 a mask 31 is employed. In the next step, anelectrically conductive material 34 is deposited within the trench andaround the trench on the bottom side 18 of the substrate body 11. Theelectrically conductive material 34 can be deposited by physical vapordeposition or by chemical vapor deposition. For example, theelectrically conductive material 34 can comprise titanium, tantalum,tantalum nitride or copper. With a mask 31 applied on the bottom side 18of the substrate body 11 the electrically conductive material 34 ispatterned and etched away in such a way that a part of the bottom side18 of the substrate body 11 is free of the electrically conductivematerial 34.

In FIG. 4K it is shown that in the next step of the method anotherisolation layer 33 is deposited on the bottom side 18 of the substratebody 11. With this, an electrically conductive via 17 is formed in thesubstrate 43. The cap body 14 is thinned in such a way that the massesof the substrate body 11 and the cap body 14 are approximately equalwhich means that the membrane 13 of the pressure sensor 12 is positionedin the plane of minimal stress of the pressure sensor device 10.

In FIG. 4L it is shown that a mask 31 is positioned on top of the capbody 14. With the mask 31 the cap body 14 is patterned in such a waythat an opening 15 is formed in the cap body 14. The opening 15 is atrench through the whole cap body 14. In this case the opening 15 ispositioned besides the membrane 13 on the pressure sensor 12 in alateral direction. This means, the opening 15 extends in a verticaldirection z, but it is not positioned directly above the membrane 13.Since the membrane 13 is already released, it is not required anymore toetch the sacrificial layer 24 and thus the opening 15 can be small andit is not necessary that the opening 15 is positioned directly above themembrane 13. If the opening 15 is small, the stress compensation withinthe pressure sensor device 10 is more well-defined than for a largeropening 15.

At next as shown with FIG. 4M a mask 31 is attached to the bottom side18 of the substrate body 11 in order to etch a part of the isolationlayer 33. With this, a part of the isolation layer 33 is etched in sucha way that a part of the electrically conductive material 34 is free ofthe isolation layer 33. With this, a bottom contact 35 is formed at thebottom side 18 of the substrate body 11 where a solder ball 38 issoldered. Therefore, the pressure sensor device 10 is surface-mountable.As a last step, the pressure sensor device 10 is singulated by dicing.

FIG. 4N shows an exemplary embodiment of the pressure sensor device 10processed by the method described with the FIGS. 4A to 4M with anadditional top layer 20 on top of the sealing layer 32. In this case thelateral extent of the membrane 13 has to be large enough to enablepressure sensing with the thicker and stiffer membrane 13 because of theadditional top layer 20 on top of the membrane 13.

FIG. 4O shows an exemplary embodiment of a pressure sensor device 10processed by the method shown with the FIGS. 4A to 4M without additionalelectrically conductive walls 29. In this case a larger amount of thetop layer 20 is etched away during the etching of the cavity 16. Thisreduces the stiffness of the connection between the cap body 14 and thesubstrate body 11 and it reduces the stability of the whole pressuresensor device 10.

With the FIGS. 5A to 5L another exemplary embodiment of the method forforming a pressure sensor device 10 is described. In this process flowthe electrically conductive via 17 is formed in the substrate 43 beforethe membrane 13 is released.

FIG. 5A shows the pressure sensor 12 on top of the substrate body 11 asdescribed for FIG. 4A.

FIG. 5B shows the deposition of the top layer 20 on top of the pressuresensor 12 as described for FIG. 4B.

In FIG. 5C it is shown that in the next step of the method the cap body14 is connected with the top layer 20. The cap body 14 and the top layer20 are for example connected via direct bonding. Advantageously, theconnection is stable at elevated temperatures, so that it is possible todeposit a final passivation or a sealing layer 32 on the membrane 13 inorder to hermetically seal the membrane 13.

As shown in FIG. 5D the substrate 43 is thinned to for example 100 to200 μm via grinding. The substrate 43 is thinned in order to introducean electrically conductive via 17 in the substrate 43.

In the next step of the method as shown in FIG. 5E an electricallyconductive via 17 is formed in the substrate 43. The electricallyconductive via 17 comprises two isolation layers 33 and an electricallyconductive material 34.

In FIG. 5F it is shown that in the next step of the method, a temporaryhandling wafer 36 is attached to the substrate 43 with an adhesive 37.By attaching a temporary handling wafer 36 to the bottom side 18 of thesubstrate body 11, the electrically conductive via 17 is protected frometching processes and mechanical damages. Therefore, advantageously thehandling wafer 36 is only removed after the membrane 13 is released. Theelectrically conductive via 17 is also protected during grinding oretching of the cap body 14. If the pressure sensor device 10 is heatedto elevated temperatures during the further processing, the adhesive 37can be a high temperature compatible material which is removed after theprocessing. It is also possible that the adhesive 37 is a hightemperature compatible material which remains on the substrate body 11.However, in this case the adhesive 37 needs to be patterned in such away that the electrically conductive via 17 can be electricallycontacted from the bottom side 18 of the substrate body 11. If onlylower temperatures are employed during the further processing theadhesive 37 can be a bonding or a tape which is compatible only withlower temperatures and which can be removed. It is also possible toemploy a permanent bonding.

FIG. 5G shows that in the next step of the method the opening 15 isintroduced in the cap body 14. Before introducing the opening 15, thecap body 14 is thinned. The opening 15 can be introduced by deepreactive ion etching of the cap body 14.

In FIG. 5H it is shown that the membrane 13 of the pressure sensor 12 isreleased. As described for FIG. 4E, the sacrificial layer 24 and the toplayer 20 are etched away through the opening 15 and the etch openings28. Since no additional electrically conductive walls 29 are introducedin the top layer 20 a part of the top layer 20 is also etched away belowthe cap body 14. Preferably, the lateral distance that the top layer 20is etched below the cap body 14 is less than 10 μm. After etching thepressure sensor device 10 is annealed at an elevated temperature inorder to remove all residues and water.

As shown in FIG. 5I in a next step the temporary handling wafer 36 isremoved. The isolation layer 33 on the back side 18 of the substratebody 11 is patterned in such a way that a part of the electricallyconductive material 34 is free of the isolation layer 33 and a bottomcontact 35 of the pressure sensor device 10 is formed.

FIG. 5J shows that a sealing layer 32 is deposited onto the membrane 13.As described for FIG. 4F, the sealing layer 32 is a compressive film.Advantageously, the membrane 13 is only released after the formation ofthe electrically conductive via 17 in order to avoid membrane fractureduring the processing and cleaning.

In FIG. 5K it is shown that the bottom contact 35 is contacted by asolder ball 38. At this processing stage the pressure sensor device 10can be tested. As a next step, the pressure sensor device 10 issingulated by for example dicing.

FIG. 5L shows the dicing step. A dicing foil 42 is arranged on top ofthe pressure sensor device 10 which means that it is attached to the capbody 14 or the sealing layer 32 on top of the cap body 14. Thisarrangement has the advantage that the membrane 13 is protected duringthe dicing and it does not have to be cleaned after the dicing step.Furthermore, because of the thickness of the cap body 14 the dicing foil42 will not stick to the membrane 13. Therefore, the dicing foil 42 canbe removed without damaging the membrane 13.

FIG. 6 shows an exemplary embodiment of the pressure sensor device 10with electrically conductive walls 29 arranged around the membrane 13.As described for FIG. 4C the electrically conductive walls 29 protectthe top layer 20 from being etched during the release of the membrane13. Here, it is shown that the electrically conductive walls 29 arearranged on top of the electrically conductive layer 25 with theadhesion layers 26 and 27. The handling wafer 36 is attached to thesubstrate body 11 by the adhesive 37 and the handling wafer 36 can beremoved during processing. Advantageously, the adhesive 37 is compatiblewith high temperatures. If the adhesive 37 cannot be removed, it isrequired that it can be patterned such that the pressure sensor 12 andthe integrated circuit 21 can be electrically contacted from the bottomside 18 of the substrate body 11.

FIG. 7 shows an exemplary embodiment of the pressure sensor device 10where the opening 15 in the cap body 14 is positioned above the pressuresensor 12 in vertical direction z, and the lateral extension of theopening 15 is smaller than the lateral extension of the pressure sensor12. A smaller opening 15 in the cap body 14 increases the mechanicalstiffness of the pressure sensor device 10.

FIG. 8 shows an exemplary embodiment of the pressure sensor device 10where the lateral extension of the opening 15 in the cap body 14 isreduced, and the sealing layer 32 covers the membrane 13 and the capbody 14. For a reduced opening 15, it is still necessary to cover thewhole membrane 13 with the sealing layer 32.

FIG. 9 shows an exemplary embodiment of the pressure sensor device 10with a reduced lateral extension of the opening 15 in the cap body 14and the sealing layer 32 coating the membrane 13 and the cap body 14. Inthis case the opening 15 is very small and the membrane 13 comprisesfewer etch openings 28 in order to guarantee that all etch openings 28of the membrane 13 are covered with the sealing layer 32.

With FIGS. 10A to 10K an exemplary embodiment of the method for forminga pressure sensor device 10 is described. In this process flow themembrane 13 is released before introducing the electrically conductivevia 17 in the substrate 43.

FIG. 10A shows the pressure sensor 12 on top of an integrated circuit 21which is on top of the substrate 43. The membrane 13 of the pressuresensor 12 is released and it is covered with the sealing layer 32.

As shown in FIG. 10B in the next step of the method a photosensitiveglue layer 39 is deposited on top of the sealing layer 32. The gluelayer 39 can be for example approximately 25 μm thick. The glue layer 39can be polyimide-based and it can withstand elevated temperatures. Atnext, the glue layer 39 is patterned by employing a mask 31 in such away that the area of the membrane 13 is not covered with the glue layer39. The glue layer 39 can be patterned by lithography.

As shown in FIG. 10C the cap body 14 is patterned or etched in such away that one or more recesses are formed in the cap body 14. Therecesses can be formed by deep reactive ion etching and they can forexample be approximately 300 μm deep. The recesses furthermore have asimilar lateral extension as the membrane 13. The patterned cap body 14is positioned on top of the glue layer 39 in such a way that a recess ispositioned above the membrane 13 in vertical direction z. The cap body14 is connected with the glue layer 39 for example at an elevatedtemperature of 250° C.

FIG. 10D shows that the substrate 43 is thinned to a thickness ofapproximately 100 to 200 μm.

As shown in FIG. 10E a trench which is formed in the substrate 43 iscoated with an isolation layer 33. The isolation layer 33 can comprisean oxide, a nitride or a polymer. As described for FIG. 4I the isolationlayer 33 is in direct contact with the back contact 30 of the integratedcircuit 21.

In FIG. 10F it is shown that the isolation layer 33 is removed on theback contact 30. At first, a resist layer 40 is deposited on the bottomside 18 of the substrate body 11. The resist layer 40 is patterned insuch a way that the isolation layer 33 is free of the resist layer 40around the back contact 30. The isolation layer 33 is removed from theback contact 30 by etching while the rest of the isolation layer 33 isprotected by the resist layer 40. At next, the resist layer 40 isremoved.

As shown in FIG. 10G in the next step of the method an electricallyconductive material 34 is deposited on the bottom side 18. For exampleat first, tantalum or tantalum nitride can be deposited by physicalvapor deposition, and in the next step copper is deposited also byphysical vapor deposition. At next, another resist layer 40 is depositedand patterned. Afterwards, another electrically conductive material 34which can be copper is deposited by electroplating through the resistlayer.

In FIG. 10H it is shown that the remaining resist layer 40 and a part ofthe electrically conductive material 34 which is not within the trenchare removed. The electrically conductive material 34 is removed by wetetching. Moreover, a further isolation layer 33 is deposited on thebottom side 18 of the substrate body 11.

FIG. 10I shows that the further isolation layer 33 is patterned in sucha way that a part of the electrically conductive material 34 is free ofthe isolation layer 33 and a bottom contact 35 of the pressure sensordevice 10 is formed. The bottom contact 35 is electrically contactedwith a solder ball 38.

In FIG. 10J it is shown that a grinding tape 41 is applied to the bottomside 18 of the substrate body 11. At next, the cap body 14 is thinned toa thickness of for example approximately 230 μm. Then, the opening 15 isintroduced in the cap body 14, such that the mass of the substrate body11 equals approximately the mass of the cap body 14.

In FIG. 10K is shown that after removing the grinding tape 41 thepressure sensor device 10 can be tested and calibrated. The pressuresensor device 10 can be electrically contacted at the solder ball 38 andthe membrane 13 can be exposed to air or other gas. Afterwards, thepressure sensor device 10 can be applied to a dicing foil 42 and it canbe singulated via dicing.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method for forming a pressure sensor device,the method comprising: providing a pressure sensor on a substrate body,the pressure sensor comprising a membrane; depositing a top layer on topof the substrate body and the pressure sensor; connecting a cap bodywith the top layer, a mass of the cap body being approximately equal toa mass of the substrate body; and introducing at least one opening inthe cap body.
 2. The method for forming the pressure sensor deviceaccording to claim 1, wherein the mass of the substrate body amounts toat least 80% of the mass of the cap body and at most 120% of the mass ofthe cap body.
 3. The method for forming the pressure sensor deviceaccording to claim 1, wherein the mass of the substrate body amounts toat least 95% of the mass of the cap body and at most 105% of the mass ofthe cap body.
 4. The method for forming the pressure sensor deviceaccording to claim 1, wherein a handling wafer is connected to thesubstrate body at a side of the substrate body facing away from the capbody by an adhesive material, and wherein the handling wafer and theadhesive material are removed.
 5. The method for forming the pressuresensor device according to claim 1, wherein a handling wafer isconnected to the substrate body at a side of the substrate body facingaway from the cap body by an adhesive material which is patternable,wherein the handling wafer is removed, and wherein the adhesive materialis not removed.
 6. The method for forming the pressure sensor deviceaccording to claim 1, further comprising etching a vertical,electrically conductive via in the substrate body before the membrane isreleased.
 7. The method for forming the pressure sensor device accordingto claim 1, further comprising etching a vertical, electricallyconductive via in the substrate body after the membrane is released. 8.The method for forming the pressure sensor device according to claim 1,further comprising forming a cavity below the membrane.
 9. The methodfor forming the pressure sensor device according to claim 1, wherein thesubstrate body comprises at least one vertical, electrically conductivevia.
 10. The method for forming the pressure sensor device according toclaim 1, wherein the pressure sensor device is surface mountable. 11.The method for forming the pressure sensor device according to claim 1,wherein the top layer and the cap body are connected via direct bonding.12. The method for forming the pressure sensor device according to claim1, wherein the opening in the cap body is positioned above the pressuresensor in a vertical direction and extends over a total lateralextension of the pressure sensor.
 13. The method for forming thepressure sensor device according to claim 1, wherein the opening in thecap body is positioned above the pressure sensor in a vertical directionand a lateral extension of the opening is smaller than a lateralextension of the pressure sensor.
 14. The method for forming thepressure sensor device according to claim 1, further comprising thinningthe cap body before introducing the at least one opening.
 15. The methodfor forming the pressure sensor device according to claim 1, furthercomprising thinning the substrate body after the cap body is connectedto the substrate body.
 16. The method for forming the pressure sensordevice according to claim 1, wherein the at least one opening isintroduced by deep reactive ion etching of the cap body.